Carbon monoxide is typically produced by catalytically reforming a hydrocarbon feed with steam, and optionally, carbon dioxide, at high temperatures. The reaction occurs in a steam methane reformer (SMR) which contains catalyst-filled tubes housed in a furnace. The synthesis gas exiting the reformer contains carbon monoxide (CO) along with hydrogen, carbon dioxide (CO.sub.2), steam and unconverted methane according to the equilibria established in the following reactions: EQU CH.sub.4 +H.sub.2 O.rarw..fwdarw.3H.sub.2 +CO Steam Reforming EQU H.sub.2 O+CO.rarw..fwdarw.H.sub.2 +CO.sub.2 Water Gas Shift EQU CH.sub.4 +CO.sub.2.rarw..fwdarw.2H.sub.2 +2CO CO.sub.2 Reforming
The above-mentioned reactions are generally carried out at high temperatures (800.degree.-1000.degree. C.) and at high pressures (5-30 atmospheres) wherein the reactants are contacted with a nickel based catalyst. These reactions are thermodynamically controlled. Therefore, the reformate effluent composition shall depend on many variables including pressure, temperature, molar ratio of steam/methane in the reactor feed and carbon dioxide concentration in the reactor feed. A typical SMR effluent composition (mole fractions) possesses 73% H.sub.2, 13% CO, 8.5% CO.sub.2 and 5.5% CH.sub.4 when the SMR reaction is conducted at 850.degree. C. and 25 atmospheres using a CO.sub.2 -free feed mixture containing a 3:1 water/methane molar ratio. The SMR effluent is subjected to a series of reaction and separation operations in order to recover a high purity H.sub.2 product (99.9+mole %) or a high purity CO product (99.5+mole %).
Carbon monoxide provided in commercial SMR plants is typically used to manufacture isocyanates and polycarbonates through phosgene chemistry. Alternatively, certain processes for producing oxoalcohols require a synthesis gas having a 1:1 ratio of hydrogen to carbon monoxide. By-product hydrogen and export steam formed during such SMR processes may have fuel value, but may not be required as products.
As is well known in the industry, synthesis gas having a high CO content is produced by injecting CO.sub.2 into the reformer feedstock and by reducing the ratio of steam to hydrocarbon in the SMR feedstock. The SMR feedstock can be further enriched in CO.sub.2 by recycling CO.sub.2 produced and separated from the synthesis gas or recovered from the furnace flue gas or by importing additional CO.sub.2 into the feedstock from an outside source. SMR feedstocks having a high CO.sub.2 to methane ratio and reduced amounts of steam inhibit the water gas shift reaction from producing additional H.sub.2 from CO and will reverse this reaction to produce additional CO from H.sub.2 under extreme reaction conditions. Some CO.sub.2 also reacts with methane in the SMR feedstock to yield syngas having a low H.sub.2 /CO ratio.
The amount of CO produced in conventional SMR processes is limited by reaction thermodynamics wherein a relatively low conversion to CO (.about.10-15%) necessitates a significant separation effort to recover the desired CO product. Numerous prior art SMR processes for producing synthesis gas are known which utilize a variety of separation cycles to recover the desired CO product from the SMR reformate effluent which typically contains a mixture of hydrogen, CO, CO.sub.2 and methane.
U.S. Pat. No. 3,986,849 discloses a SMR process for converting water and a source of methane, such as natural gas, to a hydrogen product as depicted in FIG. 1. Methane and water are introduced through line 1 into a conventional SMR reactor 2 and reacted under reforming conditions to produce a H.sub.2 -enriched reformate stream 3. Stream 3 is introduced into condenser 4 to yield steam and cooled reformate stream 6 at an intermediate temperature of 250.degree.-350.degree. C. The cooled reformate is then fed into water-gas shift reactor 7 (high temperature shift reactor, alone or in combination with a low temperature shift reactor) to convert a portion of the CO in reformate stream 6 to hydrogen by reacting CO with H.sub.2 O according to the reaction EQU (CO+H.sub.2 O.rarw..fwdarw.CO.sub.2 +H.sub.2).
The above-mentioned shift reaction plays a key role in the over-all process when hydrogen is the desired product because the shift reaction increases the hydrogen concentration and quantity in the reformate product mixture prior to separating the reformate product mixture to produce essentially pure hydrogen. Shift reactor effluent 8 is further cooled to a near ambient temperature (25.degree.-50.degree. C.) by indirect heat exchange with cooling water in condenser 9 wherein a substantial amount of water is condensed and removed from the reformate via line 10. Finally, stream 11 exiting the condenser is introduced into a hydrogen pressure swing adsorption unit (H.sub.2 -PSA) to yield essentially pure hydrogen via stream 14 and a waste gas stream 13 which can be used as fuel in the reformer.
U.S. Pat. No. 4,171,206 discloses a SMR process for converting water and a source of methane such as natural gas to simultaneously yield a high purity hydrogen product and a high purity CO.sub.2 product as depicted in FIG. 2. Methane and water are introduced through line 21 into a conventional SMR reactor 22 and reacted under reforming conditions to produce a reformate stream 23.
Stream 23 is introduced into condenser 24 to yield cooled reformate stream 26 at an intermediate temperature of 250.degree.-350.degree. C. and condensate stream (not numbered). The cooled reformate is then fed into water-gas shift reactor 27 to convert a portion of the CO in reformate stream 26 to hydrogen. Shift reactor effluent 28 is further cooled to a near ambient temperature (25-50.degree.C.) by indirect heat exchange with cooling water in condenser 29 wherein a substantial amount of water is condensed and removed from the reformate via line 30. Finally, reformate stream 31 exiting condenser 29 is introduced into CO.sub.2 vacuum swing adsorption (VSA) unit 32 wherein the reformate is separated to provide an essentially pure CO.sub.2 product stream 35. The waste gas from CO.sub.2 VSA unit 32 is introduced into H.sub.2 -PSA unit 38 via line 34 and is separated to yield an essentially pure hydrogen stream 37 and waste gas stream 36 which can be used as fuel in reformer 22. The CO.sub.2 VSA unit 32 and H.sub.2 PSA unit 36 are integrated to obtain maximum separation efficiency.
A conventional SMR process is depicted in FIG. 3 wherein water and a source of methane are introduced through line 41 into a conventional SMR reactor 42 and reacted under reforming conditions to produce a reformate stream 43. Stream 43 is introduced into a CO.sub.2 absorber/stripper 44 which contains a physico-chemical solvent which removes CO.sub.2 from the pre-cooled SMR effluent to provide stream 45 which contains essentially pure CO.sub.2 and a CO.sub.2 -depleted reformate stream 46 which is introduced into thermal swing adsorption unit 47 to remove water and remaining CO.sub.2 which is withdrawn from adsorption unit 47 via line 48. CO.sub.2 and water depleted stream 49 is introduced into cryogenic cold box 50 to yield essentially pure hydrogen stream 51, essentially pure CO stream 53 and a waste stream 52 containing CO and unreacted methane which can be used as fuel in reformer 42.
Another conventional SMR process is depicted in FIG. 4 wherein water and a source of methane are introduced through line 61 into a conventional SMR reactor 62 and reacted under reforming conditions to produce a reformate stream 63. Stream 63 is introduced into a CO.sub.2 absorber/stripper 64 which contains a physico-chemical solvent which removes CO.sub.2 from the pre-cooled SMR effluent to provide a CO.sub.2 -enriched stream 65 which may be compressed via compressor 66 and reintroduced as CO.sub.2 feed into SMR reactor 62 via line 67. CO.sub.2 depleted reformate stream 68 exits TSA unit 69 via line 71 and is introduced into cryogenic cold box 72 to yield essentially pure hydrogen stream 73, essentially pure CO stream 75 and waste stream 74 containing CO and unreacted methane which can be used as fuel in reformer 62.
U.S. Pat. No. 4,915,711 discloses an SMR process as depicted in FIG. 5. A source of methane and water is introduced through line 81 into a conventional SMR reactor 82 and reacted under reforming conditions to produce a reformate stream 83. Alternately, a CO.sub.2 stream can also be introduced into the reformer to increase CO production. Stream 83 is introduced into condenser 84 to yield water condensate stream 85 and cooled reformate stream 86 at an intermediate temperature of 30.degree.-120.degree. C. The cooled reformate is then fed into CO-VSA 87 wherein the reformate is separated to provide an essentially pure CO product stream 88 and waste gas stream 89 which can be used as fuel in reformer 82.
An alternate SMR process is depicted in FIG. 6 wherein a source of methane and water is introduced through line 91 into a conventional SMR reactor 92 and reacted under reforming conditions to produce a reformate stream 93. Stream 93 is introduced into condenser 94 to yield water condensate stream 95 and cooled reformate stream 96 at an intermediate temperature of 30.degree.-120.degree. C. The cooled reformate is then fed into CO-VSA 97 wherein the reformate is separated to provide an essentially pure CO product stream 98 and waste gas stream 99 which is introduced into a conventional polymer membrane 100 to provide waste gas stream 101 which can be used as fuel in the reformer and CO.sub.2 -enriched stream 102 which is compressed by compressor 103 and introduced into SMR reactor 92 via line 104 as additional feedstock.
Another alternate SMR process for producing essentially pure CO and essentially pure hydrogen is depicted in FIG. 7. A source of methane and water is introduced through line 111 into a conventional SMR reactor 112 and reacted under reforming conditions to produce a reformate stream 113. Stream 113 is introduced into condenser 114 to yield cooled reformate stream 116 which is fed into water-gas shift reactor 117 to convert a portion of the CO and water in reformate stream 116 to hydrogen. The hydrogen-enriched reformate 127 is passed through condenser 128 to remove water and water-depleted stream 129 is passed into H2-PSA unit 130 to provide waste stream 132 which can be used as fuel in reformer 112 and an essentially pure hydrogen stream 131. A portion of the reformate can be caused to flow into line 118 upon opening valve 117a. Such reformate is passed into condenser 119 to cool the gas and to remove water prior to being transferred by line 121 into CO-VSA 122 wherein the reformate is separated to provide an essentially pure CO stream 123 and a CO-depleted stream 124 which is optionally compressed by compressor or blower 125 and passed through line 126 to be combined with line 129 as passage into H.sub.2 -PSA 130.
Another alternate SMR process for producing essentially pure CO and essentially pure hydrogen is depicted in FIG. 8. A source of methane and water is introduced through line 201 into a conventional SMR reactor 202 and reacted under reforming conditions to produce a reformate stream 203. Stream 203 is introduced into condenser 204 to yield cooled reformate stream 206 which is fed into a CO-VSA unit 207 to provide an essentially pure CO stream 208 and a CO-depleted stream 209 which is fed into a hydrogen-PSA unit 210. The process stream is further separated in the hydrogen-PSA unit 210 to provide an essentially pure hydrogen stream 212 and a waste gas stream 211 which can be used as fuel in reformer 202.
Those of ordinary skill in the art of steam methane reforming are searching for improved reforming processes wherein conversion to the desired CO product and hydrogen product is maximized. Moreover, a process which facilitates the reaction of CO.sub.2 and hydrogen to form CO and water [reverse water gas shift reaction] would be highly desirable. Unfortunately, no prior art SMR process integrations are known in the art for directly converting CO.sub.2 and hydrogen present in a process stream to CO and water. The reverse water gas shift reaction is thermodynamically unfavorable at temperatures below 800.degree. C. and temperatures typically in excess of 1000.degree. C. are required in order to obtain moderate conversion of CO.sub.2 to CO. Thus, the reverse water gas shift reaction has not been successfully integrated into a SMR process for producing CO.
Moreover, prior art processes for conducting simultaneous reaction and adsorption steps have not achieved commercial success because product flow rates do not remain sufficiently constant and the desired products are present in unacceptably low concentrations with respect to the undesired reaction products, unreacted feedstock and purge fluids. Industry is searching for ways to improve the SMR processes for simultaneously producing CO and hydrogen.